A DAW can only load one audio (ASIO) driver.
UFX+ and UC use different RME ASIO drivers (MADIface vs older USB driver).
But you could use the UC in standalone mode and connect it via ADAT to the UFX+.
This allows you to route up to 8 audio channels from UC to UFX+ via ADAT when using single speed (44.1/48 kHz).
Please note: at higher sampling rates you loose number of channels by channel multiplexing.

You should also connect the opposite direction with a TOSLINK cable from UFX+ to UC for two reasons:
a) you need to send the UC a clock signal
b) you can also use 8 outputs of your UC this way including the phones output

I would install both ASIO drivers and keep the UC connected via USB so that it is still manageable.
Then you can open two TM FX windows, one for the UFX+, another one for the UC.
The routings you store for each TM FX session individually.

You can not route audio between the two TM FX session directly, audio needs to be routed across ADAT.
If you found the final routing on the UC, then you can simply remove the USB connection, then
the last settings will be remembered automatically by the UC (see manual ch 21, stand-alone operation).

Another option would be to sell the UC and to buy an external preamp.
This would have several advantages
a) you only need once TM FX instance
b) you get more than 2 additional mic inputs
c) if you get a bigger RME preamp like the Octamic XTC or 12Mic, then you can connect it via MADI and
it can be integrated as Auxdevice in TotalMix FX, this means, that the most important Mic settings
can be stored in the snapshots of your UFX+ and easily be recalled.
This remote control functionality you get with Octamic XTC and 12Mic by using the "MIDI over MADI" feature.
Then unused bits in the MADI protocol are being used to transfer MIDI information, which makes it then
unnecessary to connect the devices by MIDI for remote control.

Please note: the Auxdevice feature for the 12Mic is not yet implemented but promised. The 12Mic has no MIDI ports, so you would need fiber optic (OM3/OM4) cabling for MADI and a Transceiver to support Multimode.